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In January 2007, Steve Jobs introduced the Apple iPhone during his keynote address at the Macworld Conference and Expo. In its first appearance onscreen and in Jobs's hand, the phone looked like a sleek but inanimate black rectangle. Then, Jobs touched the screen. Suddenly, the featureless rectangle became an interactive surface. Jobs placed a fingertip on an on-screen arrow and slid it from left to right. When his finger moved, the arrow moved with it, unlocking the phone. To some people, this interaction between a human finger and an on-screen image — and its effect on the iPhone's behavior — was more amazing than all of its other features combined.

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